Sekishu no koe (The voice of one hand clapping) (隻手の声)

Sekishu no koe' or sekishuonjo (The sound of the one hand) was one of the representative Koans of Zen, invented by Hakuin.

Hakuin was said to have stated this while addressing Zen practitioners,

"I make a sound when I clap both my hands." "Then, please report what is the sound of one hand clapping."